You can use Escalation Steps on SLA's to configure several actions that trigger at certain percentages of SLA deadlines, including notifications. You can set the deadline percentage to 90% and have it notify the responsible resource, the manager or the responsible
resource, the reviewer, the requestor, the creator, or any other email addresses that you provide.
Service level agreements can be configured in Admin -> Tickets -> Service Level Agreements. You can add escalation steps to both the Respond and Resolve By requirements.